It refers to the act of incest, but as a positive term, like "fest" in "beerfest". The word's origin dates back to the Garden of Eden and the short period after, when the only way for the human race to pass it's genes on was, you guessed it, infest. It was a great time.

After all; nothing's like family.

"I had sex with my granddaughter last night"
"Sounds like a great party"
"No, it was more than that. It was infest!"